48v 10ah cell replacement

[Mgizen]

I'm wondering if anyone has ever replaced a cell on there lifepo4 48v battery. I had a bad cell and I was sent a replacement. I have been
reading about balancing all the cells before installing new cell. what is the recommended way about doing this? I do have a rc battery
charger that is able to charge lifepo4 cells. Is it possible to charge or discharge individual cells well still in series connection?

[Ecobully]

Can you reply back with the configuration of the pack? Is it simply a 16S1P pack? How is it put together in series (with nuts&bolts -or- soldered)? Assuming that the rest of the pack is in balance, you can take a multimeter and measure the voltage of another cell in the pack, e.g. 3.55V, and then take your replacement cell and charge it up to 3.6V and discharge it @2A down to ~ 2.8V. Next charge the recently depleted cell (let it rest for an hour for best results) to match the V of pack. Look at the how many AHrs was put back into the cell--it should be close to the rated capacity.

Mark the replacement and place it on the (+) end of the pack, if you can. You may need to come back and monitor that individual cell to see if it behaves within the specs of the other cells (and balance manually once in awhile as needed).

[Mgizen]

It's a 48v 10ah golden motor battery. 16 cell wired in series and uses nuts and bus bar to connect cells. They are 10ah cells a piece.

[Bikemad]

Check out Gary's post showing what's inside the 48V 10Ah LiFePO₄ battery to see how they are assembled.

Battery manufacturers often use double sided sticky tape in between the individual cells to hold everything neatly together, but I suspect the GM pack may simply rely on the cell interconnects combined with the outer heatshrink covering to hold the whole assembly together, but I have never needed to take my GM pack apart to find out.

All the LiPo packs I have dismantled have been constructed using double sided sticky tape and I discovered the best way of separating the individual cells was to use a length of suitable fishing line as a cheese wire to gently ease the cells apart without damaging the plastic covering.

Mark the replacement and place it on the (+) end of the pack, if you can. You may need to come back and monitor that individual cell to see if it behaves within the specs of the other cells (and balance manually once in awhile as needed).

Manual balancing is not possible without dismantling the battery pack to access the individual cells (or the balance lead connector going to the BMS board) and I can't imagine anyone doing this unless the battery has developed a problem. 

If the cell has failed because the BMS has drained it more than the other cells (which can supposedly happen over time with some BMS boards).
I would simply replace the faulty cell with the new cell fitted in the same position in the pack, as your best cell (the new one) should be better suited for coping with any additional parasitic drain compared to the other weaker (more used) cells.

Alan
 

[Ecobully]

I think you can also check individual cell voltages by locating the common (-) terminal and using it for ground. Then using your multimeter, connect to the first positive terminal (right next to your common negative terminal). It should read something like 3.6V, then move to the next cell which may read something like 7.2V, and so on. You will realize that the cells in series will have pretty much an arithmetic progression of balanced cell voltages, e.g. +3.6V.

Depending on the thickness of your balance leads, you may charge the individual cells without taking them apart via these leads. 20awg (check yours to determine charge rate) wire can safely charge @ 2A+ so you're looking at a few hours maximum to get each cell to the voltage that they need to be at. There is video on YouTube explaining how to wire your RC charger to charge through the balance leads. You can tap into your balance leads by disconnecting it from the BMS, and using paperclips + alligator clips. e.g. ground wire is usually at one end and colored differently (or black) and first cell is directly adjacent to the ground wire, and increments with each wire after that.

[Ecobully]

If double sided sticky tape is used on the pouch cells, I agree with Alan that some sort of slick thread should be used. In my experience, waxed (minted) tooth floss worked best at keeping the pouches intact without any separation from respective internal cell material (e.g. don't let foil material pull away from internal cell--this happens when you just try to pull them apart).

Many times, a lazy or ineffective BMS causes individual cells to get too far out of balance and manual intervention is required to get things back in order or else the BMS stops and may give a fault code. As such, I advised the placement of the replacement cell at one end of the pack so that it's easier to access in the case of manual intervention (cell monitoring and balancing). On the packs that I build, I usually have separate JST-XH balance taps connected to individual cell banks in addition to the ones that BMS needs and have them flush mounted to the side of battery pack case. Then I simply use the proper sized JST-XH extension to connect to a balance charger. Overkill, I know! :-p

[Bikemad]

You can tap into your balance leads by disconnecting it from the BMS, and using paperclips + alligator clips. e.g. ground wire is usually at one end and colored differently (or black) and first cell is directly adjacent to the ground wire, and increments with each wire after that.

I have an adapter made by soldering a 2 pin JST-XH connector to a Deans style "T" connector which plugs onto the output lead on my iMax B8 charger to allow single cell charging or discharging at up to 3 Amps through the balance lead connector.
Unfortunately the voltage reading on the charger is quite a bit lower than the actual cell voltage so I have to manually monitor it with a voltmeter to prevent overcharging.



I can greatly reduce the voltage difference (which is very noticeable with higher current) by adding a separate balance lead as shown on this 4 cell parallel charge unit I constructed for charging recycled 18650 laptop cells which I then use in LED torches etc.:




With this setup I can charge up to 4 cells at a time using anything up to 7 Amps charging current and it doesn't overcharge them.

Alan
 

[Mgizen]

Great thanks for everyone advice. So I have got the new cell and used my rc hobby charger set to lifepo4 to charge my new cell to match the other cells in the pack. The cell voltages were 3.350v per cell. FYI the pack is a factory GM 48v lifepo4 10ah. Once installed I hook upto factory charger and wanted to see how each cell took voltage. I found the new cell to charge up faster as first then the older cells in pack reached approx 3.6 before charge turned off. But the new cell was still only about 3.480v. I was told by Gary that cells wired in parallel are self balancing due to the connection. I hooked pack upto bike and ran motor for sometime and was pleased to see that all the cells came to same voltage around 3.4v. I left pack for around 2hrs and all cells still same voltage. I then charged pack again and had same
thing the first time the new cell being a slighty lower voltage. I'm wondering how the GM BMS balance there series pack during charging
or at rest? I have read different views on charging that keeping battery a full SOC is bad and then others say it doesn't being lifepo4 doesn't matter. I have also read that the GM bms only balances on charging and that the only way to get a pack to balance is to keep the
pack on the charger for a good 7-8hrs ?